Uptake of amino acids, particularly isoleucine, will be studied in fibroblasts grown in monolayer. Previously isolated cell lines with altered isoleucine uptake will be further evaluated. Growth conditions necessary for the selection of cells with variant uptake will be investigated. Membrane vesicles will be prepared from both normal and variant fibroblasts and analyzed biochemically. Binding and uptake of amino acids to these vesicles will also be studied.